import java.util.Scanner;

public class Test {
    public static int add(int x){
        int sum = 0;
      if(x==1){
          sum += 0;
      }
      else if (x == 2) {
          sum += 1;

      } else{
          sum = add(x-1) + add(x-2);
      }
      return sum;
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int ret = add(n);
        System.out.println(ret);
    }
}
